It is often necessary to analyze a process stream in a chemical or refining process. Typically, a process stream is analyzed by pulling a slip stream off of the process stream and then passing the slip stream through a process analyzer for analysis.
Proper conditioning of analyzer samples is the single most important maintenance consideration in an analyzer installation. Issues with the sample handling system are the largest maintenance problems with analyzers.
The basic criteria for a sample handling device is to deliver a representative sample which is compatible with the analyzer, with an acceptable response time, in a safe, reliable, and cost effective manner. A representative sample may vary from the exact composition of a sample stream within predetermined and acceptable tolerances.
Often times, contaminates and immiscible liquids, such as particulates and water, must be removed from the slip stream before passing it through the analyzer to prevent inaccurate readings and damage to the analyzer. Thus, at a minimum, most process analyzers require filtration. Because the filter must remove impurities without changing the composition of the sample, inert materials such as glass, stainless steel, ceramics, and fluorocarbon are generally used. A small filter housing designed for sample conditioning is typically used. Although the limited size maximizes filter element replacement, it is necessary to prevent excess lag time.
When filters alone fail to condition the sample, sample separators are employed in order to protect the analytical equipment from subsequent damage.
One separator type known in the art is a membrane separator. Membrane separators are devices employing a polymeric membrane. The membrane strips liquid from a sample gas. The separator directs a low flow, low pressure hydrocarbon process stream across a membrane. The hydrocarbon permeates the membrane. Liquids are repelled and exit through the return. However, common issues with membrane separators is that the membranes are typically submicron-rated, which do not tolerate particles in the process. A differential pressure greater than 15 psi will push the larger water molecules through the membrane.
Another separator known in the art is a knock-out separator. Knock-out-type separators are used to remove contaminates from gas samples. Knock-out separators reduce the flow of vapor, which allows the liquid droplets to separate by gravity. Some models of knock-out pots use baffles, which allow the vapor to impact the smooth surface and drain to the side of the enclosure. The primary objection to this type of device is that it requires relatively large bodies with limited flow, which creates excessive lag time that is prohibitive for many analyzer applications.
Another separator known in the art is a kinetic separator, which takes advantage of differing fluid densities to accomplish separation. A denser contaminant particle in a sample stream possesses a higher inertial force, rendering it less susceptible to dispersion due to pressure loss. Consequently, it continues in the flow stream while system pressure and flow path contours force the lighter components to flow toward a low pressure port above the sample outlet. Two-chamber separators use the second chamber as a polishing chamber. U.S. Pat. No. 6,444,001 to Sheffield is incorporated herein by reference in its entirety.
In a flowing process stream, the condensate and solid particulates in a gaseous sample, and the heavy immiscible liquids and solid particulates in a liquid sample, are not able to negotiate a 180 degree reversal of flow direction and will tend to remain in the fluid stream, while the lighter, more representative components will reverse direction and separate from the total contaminated stream and flow toward the lower pressure port. After separation, the kinetic separator returns the remaining sample to the original process stream, while the representative sample is sent to a polishing chamber where it will experience a second kinetic energy separation and filtration for further purification. The separator functions at full system pressure to optimize inertia while keeping the flow high, thus minimizing lag time. Unlike most conditioning devices, kinetic separators have also been found to function satisfactorily in low flow and low pressure applications with a minimal amount of lag time.
Typical kinetic separator bodies are manufactured from steel bar. However, they are very expensive to machine; cooling or heating the bodies is not practical; they require many expensive fittings; and the many fittings do not look aesthetically pleasing to the end user.
Additionally, with corrosive samples, exotic metals such as Hastelloy C and Monel are used for the separator body, in order to withstand harsh chemicals. However, doing so increases the manufacturing price by as much as five times that of a standard bar stock separator when using bar stock of the exotic metal. This price differential is due to the price of the metals, longer drilling time, and re-setup fees.
Furthermore, when slip streams are very hot, cooling the body of the separator is warranted. Cooling the body improves condensing, which helps to separate the condensables by lowering the dew point. This is particularly useful when the process is very close to saturation (i.e., 100% relative humidity). However, it is very difficult and expensive to cool a large mass of steel, as with the current monolithic bar stock bodies.
Frequently, single chamber filter housings are used in a heated sample conditioning panel. The panels are heated to prevent the sample from cooling to a temperature below the dew point. Often, unanticipated and undesirable impurities are imported with the sample. This portion of the sample is much heavier with a much higher dew point than the anticipated sample, and can create a liquid/gas dual phase product which is unacceptable to process analyzers hardware and detectors.
Furthermore, another issue with separators known in the art is that the single pore size filters typically supplied with the separators are quickly overwhelmed. Very heavily contaminated product is encountered in applications such as water and catalyst dust, causing the filters to quickly fill. Even rugged edge type stainless steel filters have limited range. Currently the solution to this problem is to utilize a bank of large sock type filters, which is expensive and prohibitive because of the long lag time, expensive filter replacement cost, man hours involved, and exposure of personnel to hazardous chemicals associated with it.
Filter housings typical in this industry have one filter. They are sometimes put in parallel to facilitate a redundant configuration to allow a filter change to occur while the other housing continues to filter the product. Even more occasionally, two housing with the same pore size filters are put in parallel to increase the surface area to handle a particularly heavy particulate loading in a given application.
However, it is often difficult to choose an appropriate pore size for a particular application. The recognized standard micron ratings used for most analyzer applications is 2 or 15 micron. With only one to choose, the decision is either a filter that will remove most of the contaminates and last an acceptable time, or a filter that will remove much more of the contaminates, but must be changed more often.
There is, therefore, a long-standing but unmet need for systems and methods for removing contaminates from a slip stream before the stream is fed into a process analyzer, in a more cost effective and time efficient manner, and in which one system could be used in a variety of applications.